Knowledge of the total hydrocarbon composition is important for several reasons. One reason is using this knowledge as input data for flow instrumentation, e.g., flow computers for wet-gas/multiphase meters.
Another reason is as input to pipeline transport capacity simulations and flow assurance. Further reasons are for optimal processing and refining, for calculating the economic value of the flow, for reservoir management, and for process troubleshooting.
The economic value of a hydrocarbon flow is directly related to the composition of the flow. The costs of transport, separation and refining the hydrocarbon flow are also determined by the composition. Thus knowing the composition as accurately as possible is an important factor for maximizing the revenue and minimizing the operational expenditure (OPEX), e.g. optimizing production, transportation, and processing for optimal regularity and profit for a given field development.
The importance of knowing the overall or total composition makes it essential to have a technique that does not depend on measurements that sometimes introduce large uncertainties.
The usual method today for finding the composition of a hydrocarbon multiphase flow is by analyzing samples of the gas and oil from points of interest in the production facility, and using flow rate information provided by gas and oil flow meters or multiphase meters to recombine the measured gas and oil compositions into the total hydrocarbon composition. The gas and oil molar volumes must also be known in order to perform this recombination.
Existing techniques are used for well testing, allocation, production and reservoir management etc. However, existing techniques has its weaknesses. Sometimes inconsistent compositions are obtained and depending on circumstances it can be difficult to pinpoint where the error is. An additional independent composition measurement would therefore be beneficial.
It is well known that flow metering is exposed to measurement errors when determining the composition and gas/oil ratio of a well-stream. The use of known methods can have large uncertainties in the flow measurements depending on the flow regime and gas/liquid fraction, water-cut etc. The accuracy of flow metering technology also depends in various degree on the flow composition and composition dependent properties like density, viscosity, permittivity etc. There is a risk that poor flow measurements are used to compute a poor composition that in turn is used in the fluid property calculations of the flow instrumentation. This may in turn lead to even poorer flow measurements.
The total composition can alternatively be obtained from bottom-hole single-phase samples, but such samples are often difficult and costly or even impossible to obtain.
Error in the composition will result in large unnecessary costs due to erroneous basis for further processing. Examples are inefficient separation and processing; inefficient use of chemicals, injection of inhibitors, de-emulsifiers etc.; poor transport capacity utilization; poor operation of multiphase flow meters, which in turn creates new processing and transport problems. In the worst case a process shutdown will be the result.
It is thus a need for an alternative or additional method for estimating the total composition of a hydrocarbon multiphase flow that does not depend on any flow instrumentation.